The debate over the British prime minister's role in deciding what should replace the 2015 millennium development goals (MDGs) has been nothing if not predictable. Critics point to David Cameron's limited credentials on global poverty reduction. Supporters highlight his willingness to defend the aid budget and host a hunger summit during the Olympics. Fair points all round, but a diversion from the real issue.

As 2015 nears, the twin challenge is to galvanise international efforts to deliver on the MDG deadline while framing an agenda that addresses the great 21st-century challenges of eradicating poverty, narrowing extreme inequalities, and combating climate change. We are failing on both fronts.

The MDGs have provided a focal point for international co-operation on poverty reduction. Despite their many design flaws, the targets made a difference. They helped focus political attention on shared goals – combating hunger, preventing child deaths, tackling killer diseases and putting children in school – and provided a reference point for campaigners.

As the 2015 deadline nears, governments and donors around the world should be building on these foundations and redoubling their efforts. They could be calculating how to bring the child survival MDGs within reach, focusing their efforts on the 70% of deaths that happen in the first year of life. With 61 million children still out of school, they could be developing strategies for accelerating progress towards universal primary education. And they could be ramping up efforts in areas that have registered little gain, including malnutrition and maternal health.

Sadly, all this has fallen on deaf ears. The MDGs are drifting off the international agenda, replaced by a dialogue aimed at producing a wishlist for the future. Instead of rallying support for a drive towards the unmet promises of 2015, UN agencies, aid donors and governments have embarked on a frenzy of post-2015 MDG reflection. This debases the currency of international development targets as a force for change.

Leadership is at the heart of the problem. The MDGs provided political leaders and campaigners with a lever for achieving change, helping to galvanise support for debt relief, increased aid and a broad array of initiatives, especially in public health. Not any more.

The current state of international co-operation on development is epitomised by the high-cost, low-impact talkshop that is the Rio+20 summit. It's a case study in what happens when you combine an absence of political leadership with vague agendas and weak coalitions for change.

None of this is to discount the case for targets – including post-2015 goals – that mobilise action to counter a deepening ecological crisis. But we also need a bridge between action to deliver on the MDGs and a debate about the future. That bridge could be built around two key pillars.

The first is a strengthened focus on inequality. There is now overwhelming evidence that MDG progress has been inhibited by the failure of governments to tackle extreme disparities in the distribution of income and access to basic services. High and rising levels of income inequality are slowing the rate at which economic growth cuts poverty. Death rates among children from poor households in much of Africa are three times those in rich households – and in many countries, the gap is widening.

Strengthening equity now would accelerate progress towards the MDGs. And the remit for the high-level panel on post-2015 goals should include the development of explicit targets for reducing disparities. One example: all countries should halve the wealth gap in child survival and school completion over five years.

The second pillar is putting money in the hands of the poor. As Homi Kharas, a Brookings Institution poverty analyst, says: "As the world has got richer and poverty numbers have fallen, so the potential cost of putting a social floor under the world's poorest people has tumbled." Lifting the 1.2 billion people now in poverty above the $1.25 threshold would cost around $41bn. That's less than 0.1% of global GDP – and shrinking.

Social protection programmes that transfer cash to poor people deliver results, cutting malnutrition, reducing poverty, improving health, and enabling parents to put their children in school. Above all, they empower poor people to manage risks and work their way out of poverty. Look at the evidence from Brazil, where social protection has contributed to a decline in extreme poverty, from 10% to 2%, and an equally dramatic decline in malnutrition.

This type of model could be developed on a global scale. Governments willing to invest 1% of GDP in targeted transfers could be made eligible for aid that would cover the additional costs of reaching everyone below the poverty line. In terms of economic growth and increased productivity, the benefits of reducing malnutrition alone would dwarf the costs.

The MDGs have brought us a long way. The challenge now is to go the extra mile. We need to keep our sights on the prize of a poverty breakthrough by 2015, while preparing the ground for a post-2015 global compact on social protection capable of delivering a poverty-free world by 2025.